Pressure assisted park servo

Abstract

A pressure assisted park servo assembly for an automatic transmission includes a servo or spool valve which receives pressurized hydraulic fluid from various sources including two solenoid valves and transmission ports. The servo valve controls two flows of pressurized hydraulic fluid to a servo assembly to place the transmission in or release it from park. The improved park servo assembly exhibits enhanced operating speed.

1. A device for initiating in a transmission a park mode of operation and an out of park mode of operation, the device comprising:
 a housing that defines a cavity; 
 a piston slidably disposed within the cavity and moveable between a first position and a second position, the piston having a first face and a second face; 
 a first chamber defined by the housing and the first face of the piston; 
 a second chamber defined by the housing and the second face of the piston; 
 a first fluid port in communication with the first chamber; 
 a second fluid port in communication with the second chamber; and 
 a biasing member disposed within the housing and in contact with the piston, 
 a valve in direct communication with the first fluid port and the second fluid port and operable to selectively provide a pressurized fluid to the first fluid port and the second fluid port, and 
 wherein when a pressurized fluid enters the first chamber from the first fluid port the piston is moved to the first position to shift the transmission into the park mode of operation and the valve provides fluid communication between the second fluid port and a vent port in order to vent a fluid from the second chamber when the park mode of operation is selected, and 
 wherein when a pressurized fluid enters the second chamber from the second fluid port the piston is moved to the second position to shift the transmission into the out of park mode of operation and the valve provides fluid communication between the first fluid port and the vent port in order to vent a fluid from the first chamber when the out of park mode of operation is selected. 
 
     
     
       2. The device of  claim 1  wherein the piston includes a piston rod that extends out from the housing. 
     
     
       3. The device of  claim 2  wherein the second face is located on a side of the piston opposite the first face. 
     
     
       4. The device of  claim 1  wherein the first chamber is hydraulically isolated from the second chamber by the piston. 
     
     
       5. The device of  claim 1  wherein the valve assembly is hydraulically actuated by a plurality of solenoids in fluid communication with the valve assembly. 
     
     
       6. The device of  claim 1  wherein the biasing member is a spring located within the first chamber. 
     
     
       7. The device of  claim 1  further comprising a valve in communication with the first chamber, the valve operable to selectively allow at least one of a fluid and a gas to enter the first chamber when the piston moves to the first position. 
     
     
       8. A system for shifting a transmission into a first mode of operation and a second mode of operation, the system comprising:
 a servo assembly comprising:
 a housing that defines a cavity; 
 a piston slidably disposed within the cavity and moveable between a first position and a second position, the piston having a first face and a second face; 
 a first chamber defined by the housing and the first face of the piston; 
 a second chamber defined by the housing and the second face of the piston; 
 a first fluid port in communication with the first chamber; 
 a second fluid port in communication with the second chamber; and 
 a biasing member located within the housing and in contact with the piston; 
 
 a valve assembly having a spool slidably disposed within a bore, the valve assembly having an inlet for receiving a pressurized fluid, a first outlet in communication with the first fluid port, a second outlet in communication with the second fluid port, and an exhaust port, wherein the valve assembly is operable to selectively provide the pressurized fluid from the inlet to the first outlet when the spool is in a first position and from the inlet to the second outlet when the spool is in a second position, and wherein the first outlet is in communication with the exhaust port when the spool is in the second position and the second outlet is in communication with the exhaust port when the spool is in the first position, 
 wherein when the pressurized fluid is communicated to the first fluid port, the pressurized fluid enters the first chamber and the piston is moved to the first position to shift the transmission into the first mode of operation, and 
 wherein when the pressurized fluid is communicated to the second fluid port, the pressurized fluid enters the second chamber and the piston is moved to the second position to shift the transmission into the second mode of operation. 
 
     
     
       9. The system of  claim 8  wherein the spool of the valve assembly is moved to the first position by activation of a first solenoid in fluid communication with the valve assembly and wherein the spool of the valve assembly is moved to the second position by activation of a second solenoid in fluid communication with the valve assembly. 
     
     
       10. The system of  claim 8  wherein the piston includes a piston rod that extends out from the housing. 
     
     
       11. The system of  claim 10  wherein the second face is located on a side of the piston opposite the first face. 
     
     
       12. The system of  claim 11  wherein the first chamber is hydraulically isolated from the second chamber by the piston. 
     
     
       13. The system of  claim 12  wherein the biasing member is a spring located within the first chamber. 
     
     
       14. The system of  claim 13  wherein the first mode of operation is a park mode of operation and wherein the second mode of operation is an out of park mode of operation.